The present invention relates to packaging integrated circuit (IC) devices and, more particularly, to a Quad Flag No leads (QFN) type device with bent leads.
QFN devices have leads that do not extend out from the device edge. That is, the leads or device contacts are generally flush with the package body. One type of QFN device is a power QFN (PQFN), which includes an exposed die paddle at the bottom of the device for more-efficient heat dissipation from the die. Note that flat no-lead devices are also known as micro lead frame (MLF) devices, micro lead frame packaged (MLP) devices, or small outline no-leads (SON) devices.
FIGS. 1A-1F show steps in the assembly of a conventional QFN device 101(1) (FIG. 1D). The assembly process includes die mounting, wire bonding, encapsulation, and singulation. Note that, conventionally, multiple devices 101 are simultaneously assembled. As used herein and unless otherwise indicated, the terms device and chip refer to a singulated, packaged IC die.
FIG. 1A shows a top plan view of partial segment 100 of a lead frame array. The lead frame array is in the form of a two-dimensional array having a number of rows and the same or a different number of columns of attached lead frames outlined by a grid, including lead frames 102(1) and 102(2). The segment 100 includes lead frames 102(1) and 102(2) and portions of adjoining lead frames of the lead frame array. The lead frame array is typically stamped from a metal sheet.
Each lead frame in the array may be a pre-plated frame (PPF), which comprises a copper core plated with a metal finish comprising, for example, nickel, palladium, lead, tin, gold, and/or silver. The plated finish is solder-wettable, where a material's solder-wettability refers to the ease of adhesion of molten solder to the material. In other words, during surface mounting, molten solder adheres easily to the plating on the lead frame.
The lead frame 102(1) comprises corresponding paddle 103(1), tie bars 104(1)—such as exemplary tie bar 104(1)(1), support bars 105(1)—such as exemplary support bars 105(1)(1) and 105(1)(2), and lead fingers 106(1)—such as exemplary lead finger 106(1)(1).
Each lead frame 102 comprises (i) four tie bars 104 supporting the die paddle 103 to the intersections of support bars 105, (ii) four support bars 105, which support the lead fingers 106 and also define the grid of the lead frame array, and (iii) 32 lead fingers 106, eight on each of the lead frame's 102 four support bars 105. Note that support bars are sometimes referred to as dam bars. The support bars 105 that are in the interior of the array—in other words, support bars that are not on the periphery of the lead frame array—are shared by adjacent lead frames 102. For example, support bar 105(1)(2) is shared by lead frames 102(1) and 102(2) and, as a result, may also be referred to as support bar 105(2)(1) of lead frame 102(2). IC dies are subsequently mounted onto die paddles 103 and electrically connected to the lead fingers 106 with a wire bonding process.
FIG. 1B shows a top plan view of the partial segment 100 comprising sub-assemblies 107(1) and 107(2), following (1) the mounting of dies 108 to the die paddles 103 of the lead frames 102 of sub-assemblies 107 and (2) the electrical connection of the dies 108 to corresponding lead fingers 106 using wire bonds 109. For example, sub-assembly 107(1) comprises die 108(1) mounted to paddle 103(1) of lead frame 102(1) and electrically connected to lead fingers 106(1)—such as lead finger 106(1)(1)—using corresponding bond wires 109(1)—such as bond wire 109(1)(1), which connects lead finger 106(1)(1) to a corresponding bond pad (not shown) on the top surface of the die 108(1). The die 108(2) is similarly mounted and wire-bonded to the lead frame 102(2) of the sub-assembly 107(2). The sub-assemblies 107 of the lead frame array are then encapsulated.
FIG. 1C shows a top plan view of the partial segment 100 of FIG. 1B, comprising sub-assemblies 107(1) and 107(2), following the encapsulation of the sub-assemblies 107 with an encapsulant 110. The encapsulant 110 may be, for example, an epoxy molding compound. The top surface of the sub-assemblies 107 comprises the encapsulant 110. Other components of the sub-assemblies 107 are shown as hidden features in FIG. 1C. Next, the sub-assemblies 107 are singulated.
FIG. 1D shows a top plan view of one of the devices 101 resulting from the singulation of the sub-assemblies 107 of FIG. 1C—such as corresponding sub-assemblies 107(1) and 107(2). The array comprising partial segment 100 of FIG. 1C may be singulated or diced by, for example, a laser or a saw. The singulation may include grinding in addition to the cutting. Aside from separating the sub-assemblies 107, the singulation process removes the support bars 105 around chips 101 and portions of the lead fingers 106—including the corresponding segments of encapsulant 110—leaving behind corresponding leads 111, such as exemplary lead 111(1)(1) that corresponds to lead finger 106(1)(1) of FIG. 1B.
FIG. 1E shows a perspective view of the device 101(1) of FIG. 1D, where the device 101(1) has been flipped over showing its bottom and two sides. Die paddle 103(1) is exposed. Each of leads 111(1) has two exposed surfaces that are flush with the surfaces of the device 101(1)—i.e., a bottom surface and a side surface. Detail area 112 includes the exposed surfaces of lead 111(1)(1).
FIG. 1F shows an enlargement of the detail area 112 of FIG. 1E. The exposed surfaces of each lead 111(1), such as lead 111(1)(1), include a lead side surface 111(1)a, such as side surface 111(1)(1)a, and a lead bottom surface 111(1)b, such as bottom surface 111(1)(1)b. Each bottom surface 111(1)b is plated with the above-described plating finish, as is the bottom surface of the die paddle 103(1). Side surface 111(1)a, however, is mostly the copper core of the lead frame 102(1) of FIG. 1A. Copper is not solder-wettable. As a result, during mounting, molten solder will adhere to bottom surfaces 111(1)b, which are plated with solder-wettable plating, but not to the side surfaces 111(1)a, which are not plated.